1. Field of the Invention
The present invention relates to a method for manufacturing a magnetoresistive element having a structure in which a sense current is supplied perpendicularly to the plane of the magnetoresistive film.
2. Description of the Related Art
In recent years, reduction in size and increase in density of hard disk drive (HDD) are rapidly improved, and this trend will be accelerated in the near future. In order to improve the density of HDD, it is necessary to reduce the width of the recording track to increase the track density. However, the decrease in track width leads to reduction in size of the recorded magnetization, i.e., in magnitude of recording signals. For this reason, it is essential to improve readout sensitivity of the magnetic head reading medium signals.
Recently, GMR heads utilizing a giant magnetoresistance effect (GMR) including a high-sensitivity spin valve film are used. The “spin-valve film” is a multi-layered film having a structure in which a nonmagnetic metal spacer layer is sandwiched between two ferromagnetic layers. In the spin-valve film, the magnetization of one ferromagnetic layer (referred to as a “pinned layer” or “magnetization pinned layer”) is pinned by an antiferromagnetic layer or the like, whereas the magnetization of the other ferromagnetic layer (referred to as a “free layer” or “magnetization free layer”) is made rotatable in accordance with an external field. In the spin-valve film, a giant magnetoresistace effect can be produced by change of the relative angle between the magnetization directions of the two ferromagnetic layers.
Conventional GMR heads including the spin-valve film have a current-in-plane (CIP) structure in which a sense current is supplied parallel to the plane of the element (referred to as a CIP-GMR element). On the other hand, much attention has been paid to a current-perpendicular-to-plane (CPP)-GMR element in which a sense current is supplied substantially perpendicular to the plane of the element because the CPP-GMR element exhibits a greater GMR effect than the CIP-GMR element. However, a metal CPP element in which the pinned layer, the spacer layer and the free layer are made of metal exhibits only a low resistance change ΔRA when a current supplied perpendicularly to the film plane, even though a giant magnetoresistance effect is obtained, because the element has a low element resistance. In order to attain increase in magnetoresistive ratio of the CPP-GMR element, there is proposed a CPP-GMR element using a nano-oxide layer (NOL) containing current paths (current confined paths: CCPs) in the direction perpendicular to the film plane as the nonmagnetic spacer layer. See, for example, JP-A 2002-208744 (KOKAI). Hereinafter, such an element will be referred to as a CCP-CPP element. The CCP-CPP element can increase both the element resistance and the magnetoresistive ratio awing to the current confinement effect.
For forming a spacer layer of the CCP-CPP element, known is a method of oxidizing an alloy layer consisting of two or more metal elements different in energy of oxidation, thereby forming current paths with a less oxidizable metal element and converting a more oxidizable metal element into an insulating layer.
According to studies by the present inventors, when a CCP-GMR element is fabricated by the above method, the resulting element has the following advantages, compared to the metal CPP element. Here, a metal CPP element having a structure of lower electrode/Ta [5 nm]/Ru [2 nm]/Pt50Mn50 [15 nm]/Co90Fe10 [4 nm]/Ru [0.9 nm]/Co90Fe10 [4 nm]/Cu [5 nm]/Co90Fe10 [1 nm]/Ni83Fe17 [3.5 nm]/Cu [1 nm]/Ta [5 nm]/upper electrode is fabricated. The element is heat-treated in a magnetic field at 270° C. for 10 hours as ordering heat treatment for PtMn to pin the pinned layer. On the other hand, a CCP-CPP element having a spacer layer of a NOL formed by natural oxidation of Al90Cu10 [0.7 nm], instead of the Cu spacer layer in the above CPP element, is fabricated. Evaluation of characteristics of the metal CPP element showed an areal resistance RA of 100 mΩμm2, an areal resistance change ΔRA of 0.50 mΩμm2, and a MR ratio of 0.5%. As described above, the metal CPP element exhibits a low MR ratio because of the low element resistance even though it has a high magnetoresistive effect. On the other hand, evaluation of characteristics of the CCP-CPP element showed an areal resistance RA of 350 mΩμm2, an areal resistance change ΔRA of 5.6 mΩμm2, and a MR ratio of 1.5%. This indicates that the areal resistance RA is favorable and the areal resistance change ΔRA is more improved than that of the metal CPP element.
However, for detection of weak signals from a medium with a high recording density of 200 to 500 Gbpsi, the CCP-CPP element fabricated by using the above method has difficulty in satisfying both the areal resistance RA and the MR ratio. Studies by the present inventors suggested that insufficient element characteristics are caused by low purity of the current paths in the insulating layer.